Although implantable medical devices are designed to be well shielded from electrostatic, magnetic and/or electromagnetic interference ("EMI"), it is still possible that electromagnetic signals may be picked up by the device, for example at the sensing point of the device, through rectification or other forms of detection. As used in this application, EMI is intended to include electromagnetic radiation, magnetic fields and electrostatic fields. It is therefore also a common practice to provide filters, such as bandpass filters, in implantable devices to suppress or attenuate any EMI that may have been picked up by the device. Other filters may include adaptive notch filers, spectral shaping filters, frequency-shift filters, and Kalman filters. However, such filters may not be completely effective in eliminating interference with device operation due to the presence of EMI. Additionally, an implantable device must be as small as possible. Accordingly, the space available within the device for filters is limited, and the complexity of the filter used in designing the device is therefore also limited.
Such EMI, if not successfully suppressed, can have serious consequences in implantable, electronically controlled medical devices. For example, in implantable cardiac devices, such as defibrillators, EMI (especially pulsed RF signals) can produce a false indication of an arrhythmia, thus prompting the device to administer unnecessary and painful therapy; or EMI can mask an arrhythmia, thus preventing the device from providing possibly lifesaving therapy. There have been recent press reports of particular susceptibility of cardiac devices to pulsed digital signals of the type associated with digital cellular phones. "Stray Signals: Clutter of Airwaves Can Block Workings Of Medical Electronics", The Wall Street Journal, Jun. 15, 1994; and "Electromagnetic Compatibility Working Group Will Develop `Action Plan` For Incorporating EMC Requirements Into FDA Premarket Review Process", M-D-D-I Reports, Sep. 19, 1994, published by F-D-C Reports, Inc.
Exposure to EMI is unavoidable in the modem world of microwave ovens, cellular telephones, radar, and high speed computers. Additionally, EMI may also result from such naturally occurring phenomena as lightning and static electricity. The problem of EMI in implantable medical devices has been addressed in the prior art by monitoring an input signal provided to the device to detect anomalous conditions that indicate the possible presence or influence of EMI.
A typical prior art implantable cardiac device receives an input signal from a sensor placed within or near a patient's heart. The input signal includes a desired biomedical signal component and, when a patient is in the presence of a strong electrostatic, magnetic or electromagnetic field, an EMI component. The EMI component may be picked up at the sensor, or it may otherwise be introduced into the signal path of the device. A detector searches for the presence of an EMI signal and allows the device to provide required therapy based on the the biomedical signal as long as an EMI signal is not detected. If an EMI signal is detected above a certain level, the assumption is made that it is interfering with operation of the device and operation of the device is inhibited until such time as the EMI signal is reduced to a tolerable level.
For example, J. Kenny, Demand Cardiac Pacer With Fast Rate For Indicating Interference, U.S. Pat. No. 3,688,776 (5 Sep. 1972) describes a cardiac pacing device that detects EMI in a feedback path, based upon signal level or rate, and in response inhibits normal device operation. The device alerts a user to the presence of EMI by operating at a user noticeable high pace frequency. See, also R. Brownlee, G. Tyers, Cardiac Pacing Apparatus With Electromagnetic Interference Protection, U.S. Pat. No. 4,091,818 (30 May 1978) which describes a cardiac pacing device that includes a first channel for a demand pacing electrode and a second channel adapted to sense EMI. Upon EMI detection, the device produces a control signal that overrides demand pacing and reduces pacing to a fixed safe operating rate until such time as the interference is no longer detected.
Other art that discloses the detection of EMI in implantable medical devices includes I. Ibrahim, Implantable Medical Devices Employing Capacitive Control Of High Voltage Switches, U.S. Pat. No. 5,178,140 (12 Jan. 1993) which describes an implantable cardiac device having a common mode switch for rejecting common mode noise; and W. Irnich, Interference Recognition Circuit in A Heart Pacemaker, U.S. Pat. No. 4,516,579 (14 May 1985) which describes a pacing device that includes a decision circuit that recognizes a discriminator output signal as either an interference signal, which is not passed to a timing circuit, or as a heart action signal, which is passed to the timing circuit.
In Cardiac Sense Amplifier With Pattern Recognition Capabilities, U.S. Pat. No. 4,913,146 (3 Apr. 1990) to DeCote, Jr., a system is described in which a microprocessor is used with a noise detection and cancellation system to provide a digitized cardiac signal which is asserted to be free of noise. Such a system would be difficult to effectively implement.
Accordingly, typical approaches to solving the problem of EMI interference with the operation of implantable medical devices immediately interrupt normal device operation when EMI is detected. However, I have discovered a technique for detecting the presence of EMI in such devices that allows the device to operate in a normal manner in the presence of tolerable levels of EMI, or that can be used to discriminate between different levels or intensities or types of EMI to determine whether such EMI is actually a threat to normal device operation. Instead of merely indicating the detection of EMI, the invention can be used to determine whether or not the EMI is actually influencing the normal operation of a device and if so, to what degree.